I've been flying an NG almost exclusively for the last 1.5 years or so. I really like the plane (except I wish I was flying a -VI with air conditioning). It is much simpler to deal with vs. a 6 handle twin in a lot of ways, also being turbocharged gives one more room to work regard OEI altitudes and performance. It seems friendly enough at blue line OEI but it can and will grow teeth if you get slow just like any other twin, as has been evidenced by fatals in the platform. I would argue given the simplicity of engine operations it is a safer product than a 6 handle equivalent, but it is certainly still possible to shut the wrong one down in a panic so it isn't pilot-proof...
The Austro-engined versions tend to be quite nose-heavy (especially if TKS is in the tank up there), so watch the W&B if you have two hefty dudes up front and nothing in back...
One thing about Diamonds in general is that as they speed up those long wings generate a lot more lift, be careful if you are hand flying right after level off and an initial trim set, and get distracted while she is still getting up to cruise IAS, because you could find yourself looking back a 350-400fpm climb at current trim setting after she picked up another 10-15 kts.
The 42 in particular is pitch-sensitive and really easy to get behind on in terms of hand flying in IMC coming out of a slower or more stable platform. In non-academic terms this is where you should spend your time with an instructor on this platform if you are instrument rated. Hand-fly it in IMC or simulated and really get some crazy turns and altitude changes and try to get some helmet fire going as well while a friendly is in the right seat. I spend 80% of my safety dollars in this and 20% in OEI (VMC Demo, Single Engine ILS, etc) in terms of ongoing instruction. It's a plane that flies just brilliantly on a GFC700 but if that thing goes Tango Uniform, or if you have a KAP-140 version it WILL go Tango Uniform, you will be happy to be proficient on hand-flying in in hard IFR (which is naturally where the autopilot will quit)...
I would say the systems in general are what a by-the-book school transition will focus on for an existing competent light twin pilot. There are electrical-issue scenarios in which your gear comes down whether you like it or not, and of course the ultimate bad day is when the engines both stop turning after exhausting backup battery resources if you were so unfortunate to have had dual alternator failures. That seems implausible yet there were actual alternator issues on the earlier planes (many still may be flying with the "dodgy" alternators as they are not cheap to replace!), so never say never...
Our running joke is the good news is if you had that sort of electrical issue and eventually both engines quit, don't worry the gear will be down for you "automatically". It's one of those things that might run through one's mind while over the north Atlantic or some other long hostile ocean crossing.
Finally, and perhaps besides IMC/Hood time hand flying with an instructor, the most important thing with this airplane is to be present when it is being fueled and be religious about testing the fuel if taking it from a school/rental op etc, although with jet fuel being heavier than avgas testing is more for contaminants and water. I've stood in 100 degree desert heat, in bitter cold, at night, or when the wife is complaining about delays, etc etc etc. No fuel goes into that plane without me being there to make damn sure it is Jet-A. Misfuelings with Avgas have already happened, I have a friend with a 42 who had his misfueled up in Northern California and by a fluke caught it on the paperwork out of the corner of his eye (his blog post about it here:
https://flyingsummers.com/2018/01/06/didnt-start-the-engines-almost-died/ ). You do not want to be the pilot who has a dual flameout on departure in this plane from a misfueling...